Gas turbine systems are widely utilized in fields such as power generation. A conventional gas turbine system includes a compressor, a combustor, and a turbine. In a conventional gas turbine system, compressed air is provided from the compressor to the combustor. The air entering the combustor is mixed with fuel and combusted. Hot gases of combustion flow from the combustor to the turbine to drive the gas turbine system and generate power.
Natural gas is typically utilized as a primary fuel for a gas turbine system. The natural gas is mixed with air in a fuel nozzle assembly in or adjacent to the combustor to provide a lean, pre-mixed air/fuel mixture for combustion. Gas turbine systems typically also require a secondary fuel that allows the system to continue to run when the primary fuel is not available. The secondary fuel is typically a liquid fuel, such as oil.
Typical prior art devices and apparatus for providing secondary fuel in a fuel nozzle assembly supply the secondary fuel as a fuel stream sprayed directly into or adjacent to a flame zone. This fuel stream is a relatively rich fuel mixture, as opposed to the relatively lean pre-mixed air/fuel mixture obtained when using the primary fuel. Consequently, the temperature of the combusted secondary fuel mixture and the resulting rate of NO formation are typically undesirably high. To lower the temperature and NOx level, water, steam, or other inert fluids are typically supplied and mixed with the secondary fuel as the fuel is sprayed into the flame zone. However, this system is relatively inefficient, and expensive. For example, an independent system must be utilized to supply the water or other fluid.
One solution for reducing the inefficiencies and expenses of the above prior art solutions is to inject a portion of the secondary fuel into an airflow upstream of the ignition source, thus premixing the secondary fuel. However, this solution may have a variety of disadvantages. For example, the premixed air/secondary fuel mixture may be relatively rich, and may encourage flashback and flame-holding within the fuel nozzle. Further, some of the secondary fuel injected into the airflow may accumulate on various surfaces inside the fuel nozzle assembly, and may cause coking on these surfaces. Coking is the oxidative pyrolysis or destructive distillation of fuel molecules into smaller organic compounds, and further into solid carbon particles, at high temperatures. Coking thus causes the deposition of solid carbon particles onto various surfaces of the fuel nozzle assembly, leading to the disruption of flow in the fuel nozzle assembly and further impairing the low emissions operation of the primary fuel.
Thus, an apparatus that provides for better pre-mixing of a secondary fuel in a fuel nozzle assembly would be desired in the art. Additionally, an apparatus for pre-mixing a secondary fuel in a fuel nozzle assembly that reduces the associated expenses and increases the associated efficiency would be advantageous. Further, an apparatus for pre-mixing a secondary fuel in a fuel nozzle assembly that prevents or reduces flashback, flame-holding, and coking in the fuel nozzle assembly would be desired.